1. Technical Field
The present invention relates to a method and an apparatus for elongating a glass base material. In particular, the present invention relates to a method and an apparatus for elongating a glass base material, using which the glass base material is processed in a heating furnace and elongated, thereby producing a glass rod having a desirable diameter.
2. Related Art
A quarts glass rod, as represented by an optical fiber preform, is manufactured by elongating a large-scale glass base material having been prepared in advance, in an elongating furnace as shown in FIG. 1 so as to generate a glass rod having a thinner diameter. The elongating furnace consists of three main mechanisms, namely, a heating furnace 1, a feeder 2, and an elongating mechanism 3. The feeder 2 hangs a glass base material 5 via a hanging connector 4, and the lower part of the glass base material 5 is linked to the elongating mechanism 3 via an elongating connector 6. While elongating the glass base material 5, the lowering speed (elongating speed) of the elongating mechanism 3 is faster than the lowering speed (feeding speed) of the feeder 2, thereby narrowing the outer diameter of the glass rod after elongation to be smaller than the outer diameter of the glass base material before the elongation. The feeding speed and the elongating speed are adequately adjusted so as to maintain the outer diameter of the glass rod constant.
There are several types of elongating mechanisms. Besides the chuck type as shown in FIG. 1, which conducts elongation while grasping the lower end of the glass base material 5, there is also a roller type that performs elongation by sandwiching the glass rod 9 elongated by the rolling rollers 7 and 8, an example of which is shown in FIG. 2. In the roller type, the elongating roller can be switched between grasping and releasing so that, when a step portion such as a joint between the dummy rod and the elongated glass rod has reached the roller, the roller can be open to pass that portion. Normally, there are two sets of elongating rollers provided so as to enable elongation while one set of elongating rollers is released.
When elongation is pursued with the chuck type (hereinafter simply “chuck elongation”), the glass rod cannot be elongated longer than the operational stroke of the elongating mechanism. Although being restricted in terms of the length of resulting glass rods, the chuck elongation has an advantage of not scratching the surface of the glass rods, as well as enabling favorable control in elongating speed. When elongation is pursued with the roller type (hereinafter simply “roller elongation”), it can realize an unlimited operational stroke of the elongating mechanism in nature, by a configuration of cutting the lower part of the glass rod lower than the elongating roller to each predetermined length. This makes the roller elongation advantageous especially when creating a long glass rod from a large-size glass base material.
When elongating a large-size glass base material by using roller elongation, the resulting glass rod tends to bend more. There are many possible reasons for this failure, aside from the original bending of the glass base material being a starting material. For example, when a set of elongating rollers are used in elongation, the direction in which the glass rod is elongated becomes unstable, to bend the glass rod. Another case is that when a once bent portion of the glass rod goes through the roller, there will be another bending created at the heated portion.
In many cases, the required accuracy in bending of an optical fiber perform, a representative glass rod, is 0.5 mm at largest (e.g. no greater than 0.2 mm) for each 1 m. One solution for alleviating the bending is to adjust the horizontal position on the XY plane of the glass base material hanging section so that the neck down position of the glass base material keeps the target position within the horizontal plane during elongation, as shown in Patent Document 1. Note that the bending can be expressed as R/L (mm/m) when the maximum fluctuation width is expressed as 2R (mm) when the both ends of the glass rod 9 are fixed on the V block 10 having an interval of 1 (m) during the rotation, as shown in FIG. 3.    Patent Document 1: Japanese Patent Application Publication No. 2005-104763
An embodiment example of Patent Document 1 adopts chuck elongation. The chuck elongation, while having an advantage of restraining the bending, is not suited for elongating large-size glass base materials or long glass base materials. On the other hand, while being suited for elongating long glass base material, the roller elongation has such a characteristic that although it can generate a glass rod with sufficiently small bending at the end of elongation, it tends to cause a large bending at the beginning of the elongation, such as no smaller than 2 mm/m and with greater occurrences. As mentioned before, when attempting to obtain a long glass rod from a large-size glass base material, roller elongation is the sole option since the chuck elongation is practically impossible. However, the conventional roller elongation technique is not able to produce products having small bending throughout the elongated glass rod. A product that bends greatly incurs additional cost for correcting the bent portion using a glass lathe.
The present invention attempts to provide a method and an apparatus for elongating a glass base material by roller elongation, using which a glass rod having a small bending is obtained.